Your shield slam usage looks good during bosses, but it seems to fall off a lot during trash.
The entire class is built around the tier set feedback loop.
Shield slam gen rage > proc thunder blast > ip/execute to spend rage > spenders proc shield slam > etc
The entire basis of your damage is basically generating and spending as much rage as possible, which drives violent outburst procs, which drives your dps.
So even on trash you should be sending shield slam when available - the wowhead guide has a great rotational breakdown but like Jakey said, rage waste is your primary offensive issue.
Outside of tracking those few items like he mentioned, I’d also recommend tracking burst of power, violent outburst, and your tier set bonus